In the past the rules would say that "you can't..." when what they really meant was "it would be unwise to...". Not so much prohibitions as serving suggestions. The exceptions to standard rules should be minimal and only where wholly justified, otherwise it just adds confusion and causes arguments.

A dropped MD on the front left (say) of a semi-trailer will probably trigger when the back half of the trailer runs past it, so it's easy to say that you "can't do it". But once you got new ammunition (e.g. radio-detonated) and modes of use, then that initial judgement starts to fall apart.

Even extreme examples like... Under-body mounted MD, madness... unless that MD is dropping radio detonated mines instead of contact detonated ones. ...SD if you have truck solids ...Oil if you don't plan on making any sudden turns... even FOJ or FCE if you don't mind being caught in your own fire.

It makes zero sense for smoke which is completely harmless.

I wouldn't treat the front four facing of trailers any different to any other facing as far as weapon mounting goes. Maybe the front underbody needs a little consideration because of the kingpin and fifth wheel interaction, but I'd hand-wave that away as an engineering problem that would be solved with sufficient application. If front underbody isn't protected from mines by the back of the tractor unit, then it is sufficiently exposed that you can drop weapons from it as well.

I get told off by some for mounting MDs & OJs/FOJs on my Car's underbodies .

Have to point out that PROXIMITY FUSED Mines don't normally arm for 2+ phases after they are dropped . Unless your moving very slowly , it shouldn't be a problem .

The Rolling Tiger Luxury from Car Warriors #4 has a SMD mounted Under . In the story it puts it to good uses in conjunction with it's Jump Jets , by dropping mines on top of it's attackers while jumping over them .... ;-)

AND Oil Jets/FOJs mounted firing 3 shots via a Weapon Timer , also shouldn't be a problem if your not maneuvering on a long piece of straight Highway .

Also if you've got one or two Oil Discharges etc mounted under , can fool a rear attacker to back way off by making them think you've carrying a full OJ/FOJ there .
Particularly so as that a Discharger mounted on Under location isn't easily observed - unlike a Back or Top Discharger which are obvious at Combat Ranges .

Retractable Drop Spike Plates should also be placeable UF on longer Vehicles .

Have to point out that PROXIMITY FUSED Mines don't normally arm for 2+ phases after they are dropped . Unless your moving very slowly , it shouldn't be a problem .

PF Mines (and radio-detonated mines) don't arm immediately, this is true. But stuff like Oil -- once it's there, it's there, and fully effective. (In the very-earliest editions of _CW_, MDs, SDs, and OJs "may only be placed on the rear of a vehicle. Any other placement would endanger the user!" [Weapon data sheet].)

How about an large Oversized Vehicle with SMDs LB+RB with Spider Mines , 2 SMDs UB with TDX Mines , SMD Back with Std Spear Mines , 2x2 MDs with Std or Napham loads Corner Mounter RB & LB corners . All Radio Controlled , with about 30-40% also having Proximity Fuses - just to keep things 'interesting' ...

With 4-5 Gunners & maybe a unupgraded Computer Gunner , there should enough Firing Actions available to detonate them in just about any combo ! *veryevil*